Preparation of anhydrides

ABSTRACT

Acetic anhydride is prepared by reacting, at elevated temperature, methyl acetate or dimethyl ether with carbon monoxide in the presence of an effective amount of a catalyst which comprises: (1) cobalt, (2) a compound containing at least one quaternary nitrogen or phosphorus atom and (3) a source of iodide or bromide wherein the atomic ratio of iodide or bromide to cobalt is from 0.5:1 to 5:1. The compound containing at least one quaternary nitrogen or phosphorus atom can optionally be generated in situ by a quaternization reaction.

This invention relates to a process for the preparation of aceticanhydride by reaction of methyl acetate or dimethyl ether with carbonmonoxide.

European patent application No. 025702 discloses a process whichcomprises reacting methyl acetate or dimethyl ether with hydrogen andcarbon monoxide in the presence of a cobalt catalyst, an amine orphosphine and iodide. Although the object of the process is to produceethylidene diacetate, acetic anhydride is produced as a coproduct insome of the worked examples. Further the catalyst systems describedcontain high halide concentrations which have the disadvantage that theycan be corrosive.

It has now been found that the reaction to form acetic anhydride can besignificantly improved by modifying the catalyst system to one of lowhalide concentration.

Thus, according to the present invention a process for the preparationof acetic anhydride comprises reacting at elevated temperature methylacetate or dimethyl ether with carbon monoxide characterised in that thereaction is carried out in the presence of an effective amount of acatalyst comprising:

(1) cobalt,

(2) a compound containing at least one quaternary nitrogen or phosphorusatom,

(3) a source of iodide or bromide wherein the atomic ratio of iodide orbromide to cobalt is from 0.5:1 to 5:1.

Preferably the atomic ratio of iodide or bromide to cobalt is from 0.5:1to 2:1.

The cobalt may be added in any convenient form, e.g. in the zero valentstate or in any higher valent form. Thus, the cobalt may be added as theelemental metal in finely divided form, or as the carbonyl, or as thecarbonate, oxide, hydroxide, bromide, iodide, chloride, lower alkoxidesuch as methoxide, phenoxide, or carboxylate wherein the carboxylateanion is derived from an alkanoic acid of 1 to 20 carbon atoms.Similarly, complexes of the metal can be employed, for example, thecobalt may be coordinated with one or more ligands selected from carbonmonoxide, halides, phosphines, arsines and trivalent nitrogen compounds.Suitably, cobalt may be added as a halide, a carboxylate salt, acarbonyl or a carbonyl halide.

The compound containing at least one quaternary nitrogen or phosphorusatom can be a salt of any convenient anion for example halide,carboxylate, tosylate and the like. The quaternary nitrogen orphosphorus atom may be supplied as more than one salt, as for examplewhere it is desirable to add small amounts of iodide or bromide whileadding larger amounts of the quaternary nitrogen or phosphorus.

The quaternary nitrogen or phosphorus compound can also be added to thereactor as two precursors, the first being a trivalent nitrogen orphosphorus compound capable of forming the quaternary compound under thereaction conditions, the second being conveniently a halide-freequaternizing agent.

The trivalent nitrogen compound can be any such compound which is ableto undergo quaternisation. Suitable trivalent nitrogen compounds includeheterocyclic amines such as pyridine, picoline, quinoline,hydroxyquinoline, methylquinoline, pyrrole, pyrrolidine, pyrrolidone andthe like or an imidazole, such as imidazole, N-methylimidazole and thelike. Suitable trivalent phosphorus compounds are of formula: ##STR1##wherein R¹, R² and R³ which may be the same or different are alkylgroups of up to 20, preferably from 1 to 8, carbon atoms or monocyclicaryl groups, or R³ may be the group: ##STR2## wherein R⁴ and R⁵ are eacha monocyclic aryl group or an alkyl group and n is zero or an integer inthe range 1 to 20. Suitable compounds having the above formula includetrimethylphosphine, tripropylphosphine and triphenylphosphine.

The quaternizing agent is conveniently a halide free-alkylating agentpreferably a methylation agent. Suitable methylating agents are offormula ##STR3## where R is hydrogen, an alkyl or aryl group or analkoxy or aryloxy group, or F or CF₃. Examples are methylp-toluenesulphonate, dimethyl sulphate, methyl sulphonic acid, andmethyl fluorosulphonic acid.

By halide-free is meant not containing chlorine, bromine or iodine.

Preferably the ratio of moles of alkylating agent to gm atoms of cobaltis at least 5:1 more preferably at least 20:1 and can exceed 50:1.

Although the molar ratio of the first precursor to the second precursormay vary over a wide range, it may suitably be in the range from 0.1:1to 5:1. It has been found that the catalyst efficiency increases as theratio approaches 1:1 from either direction. Accordingly, a molar ratioof about 1:1 e.g. from 0.5:1 to 1.5:1 is preferred.

Suitable quaternary nitrogen- or phosphorus-containing compounds whichmay be added directly or generated by the method described above arethus, for example, the salts of the organic cations N-methylpyridinium,N,N-dimethylimidazolium, N-methylquinolinium, tetramethyl phosphonium,ethyltrimethyl phosphonium, methyltripropyl phosphonium andmethyltriphenyl phosphonium.

The source of iodide or bromide can be any convenient source bothorganic or inorganic including alkali metal iodides or bromides, iodineand the like.

A copromoter in the form of a lower monocarboxylic acid may be employedwith advantage. The monocarboxylic acid is preferably acetic acid.

It will be obvious to those skilled in the art that it is possible touse starting materials which supply more than one component of thecatalyst for example cobalt iodide is a source of both cobalt andiodide. Use of such materials fall within this invention providing toatomic ratio of total iodide or bromide to cobalt is from 0.1:1 to 5:1.

The elevated temperature employed may suitably be in the range from 50°to 350° C., preferably from 80° to 200° C. and pressures can be from 400to 2500 psig preferably between 900 and 1700 psig. Hydrogen can bepresent in an amount such that the ratio of the partial pressure ofcarbon monoxide to hydrogen is at least 5:1 preferably at least 10:1.High ratios of CO:H₂ favour acetic anhydride formation over ethylidenediacetate, lower ratios favour formation of ethylidene diacetate. It ispreferable, therefore, that if possible hydrogen is absent orsubstantially absent, although small amounts of hydrogen, as found incommercial carbon monoxide sources, can be tolerated.

The process may be carried out in the liquid phase or in the vapourphase. In the case of both liquid and vapour phase operation, thecatalyst and optionally also the promoter combination may be supported,ie they may be dispersed on a conventional support material, such as forexample alumina, silica, silica/alumina, zeolites, clays, titania orzirconia. The catalyst and optionally the promoter may be applied to thesupport in conventional manner, eg by impregnation from solution. Thecatalyst concentration on the support may suitably be in the range from0.01 to 10% by weight.

The process of the invention may be carried out batchwise orcontinuously.

The invention is illustrated by the following examples.

In all the following Examples all the reactants and products except theCO and hydrogen were supplied in the liquid phase and the catalyst andpromoter were employed in solution.

EXAMPLE 1 Preparation of acetic anhydride and EDA: hydrogen present

Methyl acetate (35 g), acetic acid (15 g), methyl p-toluenesulphonate(10 g), Co₂ (CO)₈ (0.2 g), N-methylimidazole (2.3 g) and sodium iodide(0.5 g), iodide:cobalt atomic ratio 3:1 were charged to a stainlesssteel autoclave which was pressured to 100 psig H₂ and to a further 1020psig with CO at room temperature. This vessel was then heated andstirred for one hour at 130° C. At this temperature the initial totalpressure was approximately 1300 psig.

Gas Chromatographic (G.C.) analysis of the reaction mixture showed it tocontain 0.6 g of acetic anhydride and 1.0 g of ethylidene diacetate.

EXAMPLE 2 Preparation of acetic anhydride: hydrogen absent

Dimethyl ether (2.1 g), acetic acid (40 g), Co₂ (CO)₈ (0.4 g), methylp-toluenesulphonate (12 g), N-methylimidazole (4 g) and sodium iodide(0.5 g), iodide:cobalt atomic ratio 1.5:1, were charged to a stainlesssteel autoclave which was pressured to 1200 psig with CO at roomtemperature. The vessel was then heated and stirred for one hour at 150°C. At this temperature the initial pressure was 1400 psig.

Analysis of the reaction product mixture by gas chromatography (G.C.)showed it to contain 1.5 g of methyl acetate and 1.7 g of the aceticanhydride.

EXAMPLE 3 Preparation of acetic anhydride: hydrogen absent

A stainless steel autoclave was charged with a mixture of methyl acetate(35 g), acetic acid (15 g), Co₂ (CO)₈ (0.4 g), methylp-toluenesulphonate (12 g), N-methylimidazole (5.3 g) and sodium iodide(0.5 g), iodide:cobalt atomic ratio 1.5:1.

CO (1100 psig) was then added to the autoclave at room temperature. Thevessel was heated and stirred for one hour at 150° C. At thistemperature the initial pressure in the reactor was 1300 psig.

G.C. analysis of the reaction product mixture showed it to contain 2.7 gof acetic anhydride at the end of the 1 hour reaction.

One advantage of the above described catalyst systems is that they arelow in iodide and therefore less corrosive than those described in theprior art.

I claim:
 1. A process for the preparation of acetic anhydride byreacting at elevated temperature methyl acetate or dimethyl ether withcarbon monoxide characterised in that the reaction is carried out in thepresence of an effective amount of a catalyst comprising:(1) cobalt, (2)a compound containing at least one quaternary nitrogen or phosphorusatom, wherein said compound containing at least one quaternary nitrogenor phosphorus atom is added as two precursors, the first being atrivalent nitrogen or phosphorus compound capable of forming thequaternary compound under the reaction conditions, the second being ahalide-free quaternising agent, and (3) a source of iodide or bromidewherein the atomic ratio of iodide or bromide to cobalt is from 0.5:1 to5:1.
 2. A process as claimed in claim 1 characterised in that thehalide-free quaternizing agent is an alkylating agent.
 3. A process asclaimed in claim 2, characterised in that the alkylating agent is amethylating agent.
 4. A process as claimed in claim 2 characterised inthat the alkylating agent is a sulphonate or sulphate ester.
 5. Aprocess as claimed in claim 4 characterised in that the alkylating agentis methyl p-toluenesulphonate.
 6. A process as claimed in claim 1characterised in that a lower monocarboxylic acid is added.
 7. A processas claimed in claim 6, characterised in that the lower monocarboxylicacid added is acetic acid.
 8. A process as claimed in claim 1characterised in that the temperature is in the range 50° to 350° C. andthe pressure in the range 400 to 2500 psig.
 9. A process as claimed inclaim 8 characterised in that the temperature is in the range 80° to200° C. and the pressure in the range 900 to 1700 psig.
 10. A process asclaimed in claim 1 characterised in that hydrogen, if present, is in anamount such that its partial pressure is not more than 10% of the totalpressure of carbon monoxide and hydrogen.